You've got me here to provide some insights about the customisation that you can do in QuickBooks Online.
At this time, QuickBooks Online allows you to customise customer's sales forms (invoice, quotes, and sales receipts). That said, modifying the layout of the purchase order isn't available.
I can see the benefit of having this option for you and your business. Know that our developers are always gathering and considering new features to be added to the system. This way, we'll be able to improve the product and meet our customer's demands.
To stay in the loop with our latest happenings, including our program road-maps, I encourage you to visit our QuickBooks Online Blog site.
To add up, here are some links that you can scan through to guide you in modifying and sending your customer's forms for your future transactions: